The present invention relates to an adhesive composition used in articles comprising a metal layer for a predetermined purpose, such as adhesive sheets and optical disks. More particularly, it relates to an adhesive composition capable of effectively preventing the corrosion of a metal layer even when using in close contact with the metal layer. The present invention also relates to a decorative adhesive sheet having the appearance with metallic luster, wherein a metal layer contained therein can be seen through a light transmitting substrate and, more particularly, to an adhesive sheet which can prevent damage and loss of the metal layer due to the corrosion and maintain the appearance of a metal luster for a long period of time. The term xe2x80x9cadhesive sheetxe2x80x9d used in the specification of the present application includes decorative sheet, marking film (e.g. label, sticker, stripe tape, etc.), adhesive film, adhesive sheet and adhesive tape.
Adhesive sheets have previously been known comprising a light transmitting substrate and a metal layer provided on the back surface of the substrate, which has the appearance with metallic luster and is useful as a decorative sheet, wherein the metal layer can be seen through the light transmitting substrate. Such an adhesive sheet usually comprises a light transmitting substrate, a metal layer made of a metal thin film layer provided on the back surface of the substrate, and an adhesive layer provided on the back surface of the metal layer, wherein the adhesive sheet is applied to an adherend via the adhesive layer. By the presence of the metal layer, the appearance of metal luster can be easily afforded to the adherend. The metal thin film layer constituting the metal layer is usually formed by a thin film forming means such as a deposition device or a sputtering device.
For example, Japanese Unexamined Patent Publication (Kokai) No. 1-174584 discloses an adhesive sheet comprising a substrate made of a laminated film consisting of a vinyl fluoride resin layer and a coated layer having excellent printability, and a metal deposited layer provided in close contact with the side of the fluoride vinyl resin layer. In the case of this adhesive sheet, when the vinyl fluoride resin having high heat resistance and good adhesion with the metal layer is used as a bonding layer between the substrate and metal layer, thereby to bond the substrate with the metal layer, fogging of the metal layer as described above can be prevented. In this publication, a vinyl chloride resin is listed as one example of the resin of the coated layer. However, the publication does not specifically disclose how the vinyl chloride resin is bonded with the vinyl fluoride vinyl resin. Furthermore, Japanese Unexamined Patent Publication (Kokai) No. 4-35938 discloses a decorative sheet using a multi-layer film comprising a layer of a polymer alloy of polyvinylidene fluoride (PVdF) and polymethyl methacrylate (PMMA) as a substrate. That is, in the case of this decorative sheet, an outermost layer is a first alloy layer enriched with PMMA and a layer provided in close contact with a metal layer is a second alloy layer enriched with PVdF. Since the first alloy layer has comparatively good adhesion with a comparatively soft resin such as vinyl chloride resin, the above substrate can be used as the bonding layer and the vinyl chloride film is used as the outermost layer of the substrate.
Since the adhesive layer of the adhesive sheet usually contains an acrylic adhesive layer, the adhesive sheet can be easily applied to the adherend by a pressing operation in the same manner as that in the case of a normal adhesive sheet. The acrylic adhesive polymer usually comprises a unit derived from an alkyl acrylate such as isooctyl acrylate, and a unit derived from a monomer containing a carboxylic group in the molecule, such as acrylic acid.
To prevent the corrosion of the metal thin film layer, a protective layer made of a specific metal oxide is directly provided on the metal thin film layer. For example, Japanese Unexamined Patent Publication (Kokai) No. 7-197342 teaches that a thin film made of zinc oxide or tin oxide is formed as a protective layer by a deposition method. Japanese Unexamined Patent Publication (Kokai) No. 7-82626 teaches that a thin film made of yttrium oxide is formed as a protective layer by an electron beam heating method.
On the other hand, an adhesive sheet using an adhesive composition comprising an adhesive polymer, which does not substantially have a carboxylic group in the molecule and has a hydroxyl group in the molecule, and an aromatic compound is also known. For example, Japanese Patent No. 2602888 discloses an adhesive sheet comprising a substrate and an adhesive layer provided on the back surface of the substrate, characterized in that said adhesive layer is obtained by adding a heavy metal deactivator (B) and a crosslinking agent (C) to an acrylic copolymer (A) containing (a) 1-20% by weight of a hydroxyalkyl acrylate or a hydroxyalkyl methacrylate and (b) 80-90% by weight of one or more alkyl acrylates as a monomer component. A general acrylic adhesive had a problem that, when applying to a substrate made of stainless steel or iron free from a surface treatment, the acrylic adhesive reacts with the metal of the substrate by ultraviolet ray and is decomposed and deteriorated, resulting in shrinkage, floating and peeling of the film. To the contrary, in the case of the adhesive sheet, decomposition and deterioration of the adhesive by ultraviolet ray do not occur and peeling of the film does not occur. As the heavy metal deactivator (B), there can be used an aromatic compound which is liable to incorporate a metal ion to form a complex compound. Particularly preferred are those which have a hydroxyl group and a carbonyl group at the ortho-position of the benzene ring and have this carbonyl group and a NH group of triazole at the xcex1- and xcex2-positions, such as disalicyloylhydrizide decamethylenecarboxylate and 3-(N-salicyloyl) amino-1, 2, 4-triazole.
However, as described above, when the protective film is provided to prevent the corrosion of the metal layer, particularly loss of the metal thin film layer, a comparatively complicated protective film forming step must be introduced in the production step and, therefore, the production of the adhesive sheet becomes complicated.
On the other hand, as described in the above patent publications, difficulties in the production as described above can be overcome if the formation of the protective film can be omitted by using the adhesive sheet formed by using a specific adhesive. However, the above publications do not specifically disclose the adhesive composition (particularly chemical structure and content of the aromatic compound) which effectively prevents the corrosion of the metal layer and is effective to prevent loss of the metal layer when the adhesive sheet is used for a long period of time.
An adhesive composition comprising i) a hydroxyl group-containing adhesive polymer, and ii) an aromatic carboxylic acid compound, wherein said polymer comprises a sufficiently low amount of monomer units containing a carboxylic group that the adhesive composition is Corrosion Resistant.
For purposes of the present invention o an adhesive composition is considered to be Corrosion Resistant if a laminate of the adhesive on a galvanized iron plate (substrate), when dipped in a sodium chloride solution having a concentration of 5% by weight at a normal temperature (about 25xc2x0 C.) for 15 days does not exhibit corrosion up on ordinary visual inspection.